This invention relates to a centrifugal switch for a rotary shaft driven by an electric motor. More particularly, the invention relates to a centrifugal switch for opening a circuit at a predetermined speed of rotation, preferably about 200 revolutions per minute below synchronous speed of the motor. Such a switch opens the circuit to the starter windings upon the motor shaft attaining a predetermined rotational velocity, whereupon the motor continues to receive power input only through the running windings.
Heretofore, other types of prior art centrifugal switches have required that metallic contact slip rings and bushes continuously move in relative sliding inter-engagement at all times during operation of the motor to insure that the starting winding would be effective during acceleration of the motor. Thus the bushes and/or slip rings would become worn beyond use quite rapidly and the bushes could not then be maintained in positive engagement with the slip rings because of inefficiencies in sensing and controlling the rotational speed of the shaft in a predetermined manner.
Magnetically-operated centrifugal switches utilizing an attraction between opposite poles of two opposing and adjacent magnets for maintaining a circuit and relying upon centrifugal force to separate the magnets and consequently deactivate a circuit have also heretofore been known and used, but this type of switch can develop an accumulation of dirt or grime on the faces of the opposing magnetic poles over time which will detrimentally affect the performance of the switch. The magnetically-operated centrifugal switch known heretofore has a tendency, due to accumulation of dirt and grime, to activate or deactivate an associated circuit prematurely since the centrifugal force will then more readily act to separate the opposed magnets thereof.
The centrifugal switch of the present invention overcomes the problems associated with the prior art switches. Magnets are not utilized in the present switch and the slip rings and bushes do not have to move continuously in relative sliding interengagement at all times during operation of the motor. Furthermore, the switch is of a standardized construction which will fit substantially all electric motors of the same shaft diameter. In addition, while in the operative position, two brushes are always in contact with each slip ring, which will carry the load very effectively.